1. Field of the Invention
This invention relates to a transmitting apparatus and a method of adjusting the gain of a signal to be transmitted, and a receiving apparatus and a method of adjusting the gain of a received signal, and more particularly, is suitable for use in mobile radio communications systems such as cellular telephones and so on.
2. Description of the Related Art
Conventionally, in this type of mobile radio communications system, communications are made between a mobile terminal and a base station through an interconnecting radio channel. In such a mobile radio communications system, the signal level is always changing due to changes in communication distance between the mobile terminal and the base station, the influence of fading on a transmission channel, and so on.
For this reason, the mobile station is provided with a gain adjusting circuit such as an amplifier and an attenuator, such that the gain adjusting circuit absorbs fluctuating level portions in a received signal, whereby a demodulator is supplied with the received signal having its level adjusted to be constant. The transmission side of the mobile terminal is also provided with a gain adjusting circuit which adjusts a signal to be transmitted to a desired signal level such that a constant signal level is supplied to the base station.
In this way, a mobile terminal is generally provided with any gain adjusting circuit for both transmission and reception in order to avoid the influence of level fluctuations. In this event, a gain adjusting width required by the gain adjusting circuit differs from one system to another.
If a gain adjusting width required by a system extends over a range of 80 to 90 dB!, it is extremely difficult to realize this only with a single gain adjusting circuit from a technical point of view, in consideration of the isolation and dynamic range of elements. Even if it is technically possible, it is almost infeasible, taking into account the manufacturing cost.
To avoid this problem, conventionally, as shown in FIG. 1, a gain adjusting circuit for transmission is divided into a plurality of portions, in a transmitter circuit 1 in a mobile terminal. For example, if gain adjustment of 80 dB! is required by a whole system, a level change over 80 dB! must be achieved at a transmission antenna 2. Thus, 50 dB! is adjusted by an intermediate frequency (IF frequency) and the remaining 30 dB! by a radio frequency (RF frequency).
More specifically, an IF signal S1 having a constant level, fed from an input terminal 3, is inputted through an IF signal line 4 to a first variable gain amplifier 5 in which the IF signal S1 is adjusted for the gain by a width of 50 dB!. The IF signal S1 is next inputted to a frequency mixer 6 in which the IF signal S1 is subjected to a frequency conversion using a local signal S2 to be converted into an RF signal S3. The RF signal S3, after passing through a bandpass filter (BPF) 7 for removing unnecessary frequency components, is inputted to a second variable gain amplifier 8 in which the RF signal S3 is adjusted for the gain by a width of 30 dB!. Finally, the gain adjusted RE signal S3 undergoes a constant signal amplification (for example, approximately 20 dB!) by a power amplifier 9, and then is inputted through an RF signal line 10 to a bandpass filter 11 for removing herein unnecessary frequency components. The RF signal S3 having the unnecessary components removed is finally supplied to the transmission antenna 2 as a signal to be transmitted having desired power.
In the transmitter circuit 1 as described above, since the gain adjustment is divided into two steps, a signal level change from the input terminal (3) to the output terminal (2) can be limited to 50 dB! at maximum. Additionally, since this results in a smaller difference between a maximum level and minimum level of a signal at connecting points of respective elements, the dynamic range of each element can be reduced. As a result, the transmitter circuit 1 can provide a wide dynamic range as a whole while limiting the dynamic range of each element.
As another trend in recent years, the mobile radio communications system has utilized a wider frequency bandwidth with an increasing number of communication channels. In this case, however, if the BPFs 7, 11 as illustrated in FIG. 1 are to be formed of a single element, this causes inconveniences such as an extremely large physical volume of the filter, electrically large loss occurring in the pass band (in other words, required characteristics cannot be satisfied), and so on.
To avoid such inconveniences, conventionally, the pass band of a bandpass filter for a wide frequency bandwidth is divided into a plurality of regions so that the same function as a single bandpass filter is implemented by a plurality of filter elements. For example, as illustrated in FIG. 2 in which parts corresponding to those in FIG. 1 are designated the same reference numerals, in a transmitter circuit 20, the each pass band of the BPFs 7, 11 is divided into two so that a single pass band is implemented by two filter elements.
More specifically, the BPF 7 is composed of two bandpass filters 21, 22 having different pass bands from each other and switches 23, 24 for switching these filters. A desired characteristic can be obtained by changing over the switches 23, 24 in accordance with the frequency of an RF signal S3. Also, the BPF 11 is composed of two bandpass filters 25, 26 having different pass bands from each other and switches 27, 28 for switching these filters. A desired characteristic can be obtained by changing over the switches 27, 28 in accordance with the frequency of the RF signal S3. In this way, the BPFs 7, 11 each having a smaller volume and a smaller loss can be realized, even if the physical volumes and loss of the switches 23, 24 and 27, 28 are negatively evaluated. Also, a reduction in size and power consumption can be achieved in the transmitter circuit 20 as a whole.
On the other hand, to avoid the above-mentioned problem with respect to the reception, a receiver circuit 30 in a mobile terminal conventionally has a gain adjusting circuit divided into a plurality of portions, as illustrated in FIG. 3. For example, if a received signal includes level fluctuations over 80 dB! at a reception antenna 31 in the system, the receiver circuit 30 is required to adjust at least 80 dB! of gain in order to make the signal level constant at a signal output terminal 32. Thus, in the receiver circuit 30, 50 dB! is adjusted by an intermediate frequency (IF frequency) and the remaining 30 dB! by a radio frequency (RF frequency).
An RF signal S5 having level fluctuations over 80 dB! received by the reception antenna 31 is passed through a bandpass filter 33 to remove unnecessary frequency components, and subsequently inputted to a first variable gain amplifier 35 through an RF signal line 34. The first variable gain amplifier 35 applies the RF signal S5 with gain adjustment of a width of 30 dB! in accordance with a signal level. Thus, the RF signal S5 outputted from the first variable gain amplifier 35 will have level fluctuations over 30 dB!. This RF signal S5, after unnecessary frequency components are removed therefrom by a bandpass filter 36, is inputted to a frequency mixer 37, where it is subjected to frequency conversion using a local signal S6 to be converted into an IF signal S7.
The IF signal S7 is inputted to a bandpass filter 38, where nonlinear distortions generated by the frequency mixer 37 and frequency components of disturbing waves are removed, and then inputted to a second variable gain amplifier 39. The second variable gain amplifier 39 applies gain adjustment of a width of 50 dB! to the inputted IF signal S7 to make the signal level of the IF signal S7 constant. In this way, the signal output terminal 32 is supplied with the IF signal S7 having its level constant.
In the receiver circuit 30 as described above, since the gain adjustment is divided into two steps, a signal level change from the input terminal (31) to the output terminal (32) can be limited to 50 dB! at maximum. Additionally, since this results in reducing the difference between a maximum level and minimum level of a signal at connecting points of respective elements, the dynamic range of each element can be reduced. As a result, the receiver circuit 30 can provide a wide dynamic range as a whole while limiting the dynamic range of each element.
In recent years, as another movement, the mobile radio communications system tends to utilize a wider frequency bandwidth with an increasing number of communication channels. In this case, however, if the BPFs 33, 36 as illustrated in FIG. 3 are to be formed of a single element, this causes inconveniences such as an extremely large physical volume of the filter, electrically large loss occurring in the pass band (in other words, required characteristics cannot be satisfied), and so on.
To avoid such inconveniences, conventionally, the pass band of a bandpass filter for a wide frequency bandwidth is divided into a plurality of regions such that a single bandpass filter is implemented by a plurality of filter elements. For example, as illustrated in FIG. 4 in which parts corresponding to those in FIG. 3 are designated the same reference numerals, a receiver circuit 40 has a single bandpass filter implemented by a two filter elements for a pass band divided into two.
More specifically, a BPF 33 is composed of two bandpass filters 41, 42 having different pass bands from each other and switches 43, 44 for switching these filters. A desired characteristic can be obtained by changing over the switches 43, 44 in accordance with the frequency of an RF signal S5. Also, the BPF 36 is composed of two bandpass filters 45, 46 having different pass bands from each other and switches 47, 48 for switching these filters. A desired characteristic can be obtained by changing over the switches 47, 48 in accordance with the frequency of the RF signal S5. In this way, the BPFs 33, 36 each having a smaller volume and smaller loss can be realized, even if the physical volumes and loss of the switches 43, 44 and 47, 48 are negatively evaluated. Additionally, a reduction in size and power consumption can be achieved in the receiver circuit 40 as a whole.
The transmitter circuits 1, 20 as mentioned above have a problem that the variable gain amplifiers 5, 8 and the power amplifier 9 must be always supplied with electric power for the gain adjustment, so that electric power is consumed for nothing.
Particularly, since the power amplifier 9 is an amplifier at the RF stage, its power addition efficiency generally tends to significantly degrade if a low level signal is inputted thereto. Thus, even if the variable gain amplifier 8 performs the gain adjustment, there is a fear that the power amplifier 9 would further consume electric power for nothing.
If electric power is consumed for nothing in this way in the transmitter circuits 1, 20, a communication available time will be shortened in a battery driven mobile terminal, thus causing a grave problem.
Similarly, the receiver circuits 30, 40 as mentioned above have a problem that the variable gain amplifiers 35, 39 must be always supplied with electric power for the gain adjustment, so that electric power is consumed for nothing. If electric power is consumed for nothing in this way in the receiver circuits 30, 40, a stand-by time and a communication available time will be shortened in a battery driven mobile terminal, thus causing a grave problem.
Also, in the receiver circuits 30, 40, if disturbing waves are present in a received signal (S5) in a pass band of the BPF 33, the variable gain amplifier 35 or the frequency mixer 37 may be saturated depending on the level of the disturbing waves, possibly resulting in restraining the received signal to cause significant degradation in the reception sensitivity.